Apparatus and method for enhancing and analyzing signals from a continuous non-invasive blood pressure device

ABSTRACT

A system and method of enhancing a blood pressure signal is disclosed. The volume of an artery in a finger is measured by a photo-plesthysmographic (PPG) system, which produces a PPG signal. This PPG system is placed inside a cuff, and the cuff pressure is controlled by the PPG signal. The portion or component of the PPG signal having a frequency higher than a predefined threshold frequency is then modified or enhanced, such as by multiplying the high frequency component by a calibration factor. A blood pressure signal is then calculated using the cuff pressure and the modified PPG signal. A blood pressure contour curve may then be generated, and a variety of parameters may be calculated using the curve.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a non-provisional of U.S. provisional patentapplication Ser. No. 61/256,081 filed Oct. 29, 2009, the entire contentsof which are incorporated herein by reference. The present applicationis also a non-provisional of U.S. provisional patent application Ser.No. 61/256,110, the entire contents of which are incorporated herein byreference.

The present application is related to U.S. patent application Ser. No.12/915,572, now U.S. Pat. No. 8.343,062, filed Oct. 29, 2010, entitled“Digital Control Method for Measuring Blood Pressure” the entirecontents of which are incorporated herein by reference.

BACKGROUND

1. Field

The invention relates generally to a method of measuring blood pressure,and more particularly to a continuous non-invasive arterial pressure(CNAP) measurement where the blood pressure signal is enhanced.

2. Description of Related Art

Pulse contour analysis (PCA) is the process of calculating parametersfrom a blood pressure pulse, especially from the contour of the pulsewave. PCA begins with measuring blood pressure (BP).

Blood pressure may be measured in a number of ways. As one example, astandard non-invasive sphygmomanometer (NBP) may be placed on the upperarm or wrist. The NBP applies pressure to the arteries, causing them toconstrict and limit blood flow. As the pressure is released, blood flowis restored in the artery, and the systolic and diastolic bloodpressures may be measured. NBP measures BP intermittently and notcontinuously, so it cannot be used for PCA.

Another device for measuring blood pressure is a finger cuff having aninfrared light source and a light detector for measuring aphoto-plethysmographic (PPG) signal that is known also from pulseoximetry. This PPG-signal is fed into a control system, which produces acounter pressure in the finger cuff. It is well known that the counterpressure equals intra-arterial pressure when the PPG-signal is keptconstant. Thus, the counter pressure, which is indirect equivalent tointra-arterial pressure, is measured. This method is known as “VascularUnloading Technique,” and the continuous pressure signal can be used forPCA.

Invasive devices may also be used to measure blood pressure, such as anintra-arterial catheter, for example. Intra-arterial transducers haverelatively high frequency transmission (up to 200 Hz) and can thereforebe used for PCA.

Some example parameters that may be calculated from the contour of thepulse wave include stroke volume (SV), cardiac output (CO), strokevolume variation (SVV), pulse pressure variation (PPV), and totalperipheral resistance (TPR). In addition, PCA can be used for othermeasurements which give insight to the human vascular properties, forexample arterial stiffness. Thus, it is desirable that the measuredblood pressure signals be as accurate as possible.

Invasive devices have the disadvantage of being overly disturbing andpainful to the patient, whereas signals from non-invasive devices haveproblems with the fidelity or accuracy of the signal.

SUMMARY

A system and method of enhancing the blood pressure signal fidelity isdisclosed. In one embodiment, a method for determining a blood pressurecontour curve includes placing a photo-plesthysmographic (PPG) systemover an artery in a human finger, the PPG system producing a PPG signalbased on volume of the artery, the PPG system including at least onelight source and at least one light detector, modifying a component ofthe PPG signal having a frequency higher than a predefined thresholdfrequency, and calculating a blood pressure signal using the modifiedPPG signal.

In another embodiment, a computing device for determining a bloodpressure contour curve is disclosed. The computing device includes apressure cuff adapted to be placed over an artery in a human finger, thecuff including a PPG system having at least one light source and atleast one light detector, a pressure sensor, and a controller forcontrolling the pressure in the cuff. The PPG system produces a PPGsignal based on volume of the artery, and a pressure signal iscalculated using the PPG signal and this pressure signal is applied tocuff and finger. The computing device modifies a component of the PPGsignal having a frequency higher than a predefined threshold frequencyand calculates a blood pressure signal using the cuff pressure and themodified PPG signal.

In yet another embodiment, a method for eliminating undesired signalcontent of a continuous non-invasive arterial blood pressure device isdisclosed. The method includes placing cuff having aphoto-plesthysmographic (PPG) system over an artery in a human finger,the PPG system producing a PPG signal based on volume of the artery,eliminating from the PPG signal an undesired portion of the PPG signal,and reconstructing the PPG signal from the remaining portion of the PPGsignal.

BRIEF DESCRIPTION OF THE FIGURES

An exemplary embodiment of the present invention is described hereinwith reference to the drawings, in which:

FIG. 1 shows a prior art Vascular Unloading Technique (VUT) controlsystem using a photo-plesthysmographic (PPG) system controlling the cuffpressure for measuring blood pressure;

FIG. 2 describes the transfer function between PPG-signal v(t) atdifferent constant cuff pressures;

FIG. 3 shows an example pulse of the remaining PPG-signal v(t) in search(open loop) and measuring (closed loop) mode;

FIG. 4 shows a block diagram using different frequency ranges withdifferent control gains and concepts;

FIG. 5 is a block diagram of the calibration method;

FIG. 6 shows the change in the remaining PPG-signal v(t) caused due tovasoconstriction of the artery;

FIG. 7 describes prior art Pulse Contour Analysis (PCA) having one timevarying input signal and several input parameters;

FIG. 8 is a block diagram of the new PCA-method and device; and

FIG. 9 shows an example computing device that may be used with thesystem and method of the present application.

DETAILED DESCRIPTION

A system and method of measuring and enhancing blood pressure (BP)signals is described. These modified, more accurate signals may then beused to more accurately calculate a variety of parameters for a patient,such as stroke volume (SV), cardiac output (CO), total peripheralresistance (TPR), and arterial stiffness, for example. The methodextracts the AC-component of the photo-plethysmographic (PPG) signal ofknown “Vascular Unloading Technique” (VUT). In combination with themeasured pressure signal, this signal is used as a second input forPulse Contour Analysis (PCA).

FIG. 1 shows a typical VUT system 100 and its control principle. The VUTsystem 100 is used to obtain a PPG signal, which can then be used tocontrol the cuff pressure, which is equivalent to the continuousarterial blood pressure. The VUT system 100 includes a“photo-plethysmographic” (PPG) system located within a finger cuff 102and having one or more light sources 104 and one or more light detectors106. The PPG-signal is fed into a control system 114 that produces apressure in the cuff 102.

In operation, a human finger 108 is placed in the finger cuff 102. Thefinger cuff 102 measures blood volume in an artery 110 of the finger108. During systole, when blood volume increases in the finger 108, acontroller 114 increases the pressure of the finger cuff 102,p_(cuff)(t), until the excess blood volume is squeezed out by pressureof the cuff. On the other hand during diastole, the blood volume in thefinger is decreased, and therefore the controller 114 decreasesp_(cuff)(t) so the overall blood volume in the artery remains constant.As blood volume and thus v(t) is held constant over time, the pressuredifference between cuff pressure p_(cuff)(t) and intra-arterialpressure, p_(art)(t), is zero. Thus, p_(art)(t) is equal to cuffpressure p_(cuff)(t), which can easily be measured by means of amanometer (pressure measuring instrument), for example. Thus,intra-arterial pressure p_(art)(t) itself is measured indirectly, and aPPG-signal v(t), which reflects the arterial blood volume changes in themeasuring area (e.g. the finger) is obtained. As the PPG signal is keptconstant, the counter pressure eliminates the arterial blood volumechanges and the diameter of the artery is also constant. Thus, arterialinflux is guaranteed during measurement, whereas venous return from thefingertip is slightly reduced.

This indirect measurement may not be accurate for a number of reasons.For example, v(t) is not truly constant since the pressure in the cuffmay not instantly track the pressure in the artery. Thus, as the cuffpressure tracks the pressure in the artery, v(t) takes on an alternatingcurrent (AC)-like component (referred to as v_(AC)(t)). VUT methods relyon their valve systems as they are producing the pressure signal.Typically these valves systems are limited to upper cut-of frequenciesof 15-40 Hz. Thus, the counter pressure in the cuff p_(cuff)(t) is oftenslower than the signal origin, which produces v_(AC)(t). Additionalfactors like pressure coupling from cuff to tissue, air supply from pumpto valve system and from valve system to cuff etc. limit the controlsystem. These factors limit VUT and lead to remaining v_(AC)(t).

Additionally, pulse pressure depends on the control loop gain(s) thatare either calculated from the maximum PPG signal amplitude v_(max)(t)according to the “PhysioCal” criteria or chosen empirically. These gainscannot be infinity, which would be necessary for zero v_(AC)(t). Whencalculated from v_(max)(t), the controller gain could be suboptimal.

The underlying mechanism between p_(cuff)(t), p_(art)(t) and v_(AC)(t)is shown in FIG. 2 for constant cuff pressures (p_(C1), p_(T), p_(C2))(lines 2 b, 2 a, 2 c, respectively). A typical S-shaped p-v transmissioncurve produces different PPG-signals v(t) depending on p_(cuff). It iswell known that the amplitude of v_(AC)(t) depends on p_(cuff) and ishighest at p_(cuff)=mean BP. There are different shapes of v(t) atdifferent p_(cuff).

Note the inverted characteristic of the PPG signal. The light from lightsource 104 is absorbed by blood. The more blood that is inside thefinger (e.g. during systole), the less light is shone through the fingerand detected by the light detector 106.

True mean BP is calculated as follows (for analog signals and timeseries):

$\begin{matrix}{{{mean}\;{BP}} = {{\frac{1}{T} \cdot {\int_{t = 0}^{T}{{p_{art}(t)}{\mathbb{d}t}}}} = {\frac{1}{N} \cdot {\sum\limits_{i = 0}^{N - 1}p_{i}}}}} & (1)\end{matrix}$where T is the pulse interval [sec] and N is the number of samples p_(i)of the beat.

A constant p_(cuff) is used in search modes of the VUT device fordetecting mean BP before the actual measurement starts. p_(cuff), wherePPG amplitude v_(AC)(t) is at maximum, represents mean BP. This startingp_(cuff) is the so called starting setpoint p_(T0).

During measuring mode the loop of the control system is closed, whichmeans that p_(cuff) is alternating with respect to v(t) and depending oncontroller gain g. According to the VUT-principle, the amplitude ofv_(AC)(t) is decreasing to a minimum Ideally v_(AC)(t) is zero, but thisis not possible since the gain is a real value and not infinity, and thevalve cut-off frequency.

FIG. 3 shows the mechanism p_(cuff) that is alternating around setpointp_(T) and is controlled by v(t). The control condition is to keep v(t),and therefore blood volume in the finger, constant. This can only bedone to a minimum amplitude of v_(AC)(t). Note the invertedcharacteristic of the control system. An increase of v(t) lowersp_(cuff) and a decrease of v(t) increases p_(cuff) due to the invertedcharacteristic of the PPG signal.

In some embodiments, it may be advantageous to have more than onecontrol loop. FIG. 4 shows a block diagram of such a control system. Inthis typical embodiment, v(t) is split into different frequency ranges.Pulsatile v_(AC)(t), low frequency v_(LF)(t) and very low frequencyv_(VLF)(t) are obtained with filters having cut-off frequencies atf_(VLF) and f_(LF). It is a further advantage that the three frequencyranges have different gains g_(AC), g_(LF) and g_(VLF). This allows foroptimal gain application to v(t).

The remaining pulsatile PPG signal v_(AC)(t), but also other frequencybands of v(t), and the state variables of the control system (e.g.gains, cut-off frequencies, etc.) can be used for a multivariatetransfer function T, which can be used for enhancing the measuredp_(cuff)(t) to p₊₊(t). Equation (2) is a more general formula, whenusing n control loops:p ₊₊(t)=p(t)+T[v ₁(t), v ₂(t) . . . v _(n)(t); g ₁ , g ₂ . . . g _(n) ;f _(C1) ; f _(C2) . . . f _(Cn)]  (2)It has been shown that frequency ranges below 0.1 Hz do not contributeto p₊₊(t). Equation (2) for the embodiment described in FIG. 4 will besimplified as follows, because only v_(AC)(t) and v_(LF)(t) contributeto a meaningful signal:p ₊₊(t)=p(t)+T[v _(AC)(t),v _(LF)(t); g _(AC) ,g _(LF) ; f _(LF) ,f_(VLF)]  (3)A linear function can be used when the correct setpoint is applied. Ascan be seen in FIGS. 2-4, at the correct setpoint, p_(T) is the point ofmaximal slope and therefore maximal pulsatile v_(AC)(t), low frequencyv_(LF)(t) and very low frequency v_(VLF)(t) is reached. Linearinterpolation can be approximated:p ₊₊(t)=p(t)+T[v _(AC)(t)·g _(AC) v _(LF)(t)·g _(LF)]  (4)where T indicates the remaining transfer function after linearinterpolation. In one example, T can be a vector of different scalingfactors between the different linearized v(t)* gain multiples.

Due to physiological reasons, pulse wave form is different when measuredat different sites (e.g. finger, upper arm, wrist, leg, etc.). Thus,because blood pressure measurement in the finger artery 110 is differentfrom blood pressure measured at other areas of the human body, fingerarterial pressure devices lack accuracy in comparison to standarddevices.

One method of enhancing the VUT pressure signal p_(cuff)(t), and thusincreasing accuracy of the signal, is to calibrate the signal v that ismeasured at the finger to a standard upper arm sphygmomanometer (NBP).One reason for doing this is that there are inherent physiological andhydrostatic differences of BP measured at the finger artery as opposedto the upper arm, since the upper arm is almost close to heart levelwhereas the finger can be anywhere. Additionally, pulse pressure (PP) ofBP depends on the control-loop gain(s) and these gains are parametersfrom the control system and not physiological. When the gain isdetermined from the maximum v_(AC)(t) amplitude according to the“physiocal” criteria, this amplitude depends on the actual vascular tone(vasoconstriction or vasodilatation). This has no information about BP.When the gain(s) are chosen empirically by increasing the gain until thesystem start to swing with resonance frequency, these gain(s) alsodepends on vascular tone and system conditions. Again, this has noinformation about BP.

The maximum v_(AC)(t) amplitude indicates only that the constant cuffpressure in search mode is equal to mean BP. The value itself is more orless a “house number” as it depends on the actual vascular tone(vasoconstriction or vasodilatation) and therefore depends on the stateof the autonomic nervous system of the patient to be measured.

Calibration methods include transforming the signal along a straightline:p ₊₊(t)=k*p _(cuff)(t)+d  (5)where k and d can be calculated from NBP-values as follows:

$\begin{matrix}{k = \frac{{SBP} - {DBP}}{{sBP} - {dBP}}} & (6) \\{d = {{SBP} - {k \cdot {sBP}}}} & (7)\end{matrix}$where SBP and DBP are systolic and diastolic values measured from theNBP calibration device (e.g., the upper arm blood pressure cuff) and sBPand dBP are systolic and diastolic values measured from the uncalibratedfinger cuff.

This method lacks accuracy because slope k is not only scaling BP-pulse,but also hypo- and hypertensive episodes. This BP-trend does not need anartificial amplification as mean BP is correctly detected by theimproved VUT system. High k values overestimate BP trend, e.g., with ak=2, a drop of BP of 40 mmHg would be displayed as 80 mmHg. Evennegative values could be displayed with such method. In addition, thismethod amplifies natural rhythms of BP, e.g., the 0.1 HzTraube-Hering-Mayer waves, and makes them look very non-physiological.

FIG. 5 shows a method that further improves the accuracy of the signalmeasured by the PPG system. The method includes only multiplying thecomponent of the signal that has a frequency content higher than somethreshold value, e.g., 0.3 Hz., by slope k. Signal components lower thanthe cut off frequency remain unamplified. In addition, the offset d isadded. Thus, the amplification formula reads as follows:p ₊₊(t)=k*p _(AC)(t)+p _(LF)(t)+d  (8)where p_(AC)(t) is the component of the measured pressure that has afrequency greater than the threshold frequency, and p_(LF)(t) is thecomponent of the measured pressure that has a frequency less than thethreshold frequency.

Pulse wave frequency content is per se higher than the actual pulse rateor pulse frequency. For a normal pulse rate of 60 beats per minute, thepulse frequency is 1 Hz and this frequency will come down to 0.5 Hz inhumans (30 beats per minute).

When a transfer function is applied in order to transfer the wave formof the pulses (e.g., from finger to upper arm wave forms) this transferfunction starts with its frequency range at the lowest possible beatfrequency, which is at approximately 0.3 Hz. Below that, the transferfunction can be constant. It would be of further advantage if thattransfer function depends on pulse frequency. This can be achieved bynormalization to heart beats instead of seconds.p _(brach)(t)=T _(norm)(p++(t))=p _(brach)(t)=T _(norm)(k*p _(AC)(t)+p_(LF)(t)+d)=p _(brach)(t)=p _(LF)(t)+d+T _(norm)(k*p _(AC)(t))  (9)

As can be seen from equation (9), only the pulse frequency contentp_(AC)(t) has to be transform as T_(norm) is constant (e.g. 1) for lowerfrequencies. This algorithm could be part of the PCA-method and computedwithin the invented device.

Another problem with known methods of detecting and enhancing VUTsignals is that the underlying PPG system cannot detect changes in theblood volume due to vasoconstriction or vasodilatation (vasomotoricchanges), which may be caused by drugs, for example. In other words,present systems cannot distinguish between the change of v(t) caused byvasoactivity as opposed to actual blood pressure changes. Thus, tofurther enhance the BP-waveform, an algorithm may used to detect changesin the blood vessel (e.g. in the finger artery) due to vasomotoricchanges. The algorithm enhances the BP frequency band, where vasomotoricactivities are active—in the very low frequency (VLF) band below 0.02Hz. This VLF-band is below Traube-Hering-Mayer waves (0.1 Hz) andbreathing frequency (appr. 0.2 Hz). Note that in this document bothTraube-Hering-Mayer waves and breathing frequency are called LF-band asboth physiological frequencies are treated within the so-called LF-loop.

The VLF-band is disturbed by vasomotoric changes coming fromphysiological- or drug-induced vasoconstriction or vasodilatation. FIG.6 shows typical changes of v(t) due to vasoconstriction which isindicated with a new S-shaped transfer function. p_(cuff) stays atsetpoint p_(T1), although setpoint p_(T2) would be correct. Theamplitude is decreased, but vasoconstriction produces also a moreremarkable change in waveform. This behavior is used for reconstructingthe VLF-band.

These vasoactivities may cause physiological BP-changes. The BP-signalis enhanced by elimination and reconstruction of VLF-band. The algorithmstarts with its functionality when the control loop is closed afterfinding the starting setpoint p_(T0) and determining at least one gainfactor for at least one control loop in searching mode. p_(T0) is equalto the actual mean BP.

As already described, the gain of the control system cannot be infinityand therefore v_(AC)(t) is not zero. Thus, p_(cuff) is not exactly equalto p_(art). If v_(AC)(t) is negative (systolic part), p_(cuff) isfollowing p_(art) (p_(cuff)<p_(art)). When v_(AC)(t) is in its positive(diastolic) halfwave, p_(cuff) is leading p_(art) (p_(cuff)>p_(art)).

Consider an example in which gain(s) were set to zero. In thissituation, which can be seen in FIG. 2 a, p_(T0) and p_(cuff) are atmean BP and v_(AC)(t) has its maximum amplitude. The area under thenegative curve equals the area under the positive half wave of the beat.Thus, p_(art) is as often greater as lower in comparison to p_(cuff)that indicates mean BP. This indicates that the setpoint p_(T0) iscorrect. Therefore, this phenomenon can be used for setpoint tracking.

When the negative and the positive half wave of an alternating signalare equal the following formula is true:

$\begin{matrix}{{\int_{t = 0}^{T}{{v_{AC}(t)}{\mathbb{d}t}}} = 0} & (10)\end{matrix}$

When this integral of v_(AC)(t) over a beat is not zero, the waveform ischanging. FIG. 2 describes this phenomenon: 2 a shows that the positiveand negative half-waves of v_(AC)(t) are equal, 2 b shows the signalwith low setpoint and a greater negative half-wave, and 2 c with highsetpoint and greater positive half-wave.

Equation (10) calculates control deviation P_(n) for the n^(th) beatthat indicates setpoint changes:

$\begin{matrix}{P_{n} = {\int_{t_{n - 1}}^{t}{{v_{AC}(t)}{\mathbb{d}t}}}} & (11)\end{matrix}$

-   where:    -   P_(n)=0->setpoint correct    -   P_(n)<0->setpoint to low    -   P_(n)>0->setpoint to high

This phenomenon is also true when gain(s) are not zero, p_(cuff) leadsand follows p_(art) and v_(AC)(t) is minimized. This phenomenon is alsotrue when the s-shaped p-v transfer function is changed due tovasomotoric changes.

Proportional control deviation P is now used for reconstruction of theVLF-band. For that, it needs also an integral part I and the newsetpoint for the n^(th) beat is as follows:

$\begin{matrix}{p_{Tn} = {p_{T\; 0} + {g_{I} \cdot {\sum\limits_{0}^{n}P_{n}}} + {g_{P} \cdot P_{n}}}} & (12)\end{matrix}$

Control loop gains g_(I) and g_(P) are determined in accordance with thegain g_(AC) for the pulsatile part and in accordance to physiologicalrhythms.

This tracking (or reconstruction) algorithm allows for the eliminationof the VLF-band with a high pass filter (e.g. digital filter). Allfrequencies content below 0.02 Hz (for example) are eliminated—only theLF-band and the pulsatile AC-component is used. Note that v_(AC)(t) iscalculated by subtracting v_(VLF)(t) from the measured PPG signal v(t)and not by subtracting the DC-component of the signal v_(DC).

FIG. 7 shows a prior art PCA having one single time varying inputsignal, either an intra-arterial catheter or a non-invasive device, andseveral input parameters. When using VUT for PCA, p_(cuff) is not equalto p_(art). This is indicated by the remaining PPG-signal v_(AC)(t). Inaddition, state variables of the control system indicate vasomotoricchanges. The remaining information may also be used to enhancePCA-algorithms.

Standard PCA methods cannot be used as they must be extended withadditional input signal(s) like v_(AC)(t) but also signals fordetermining setpoint p_(T) (which is equal to mean BP). A meaningfulsignal could be P_(n) that indicates if the setpoint must be correcteddue to vasomotoric changes. Further state variables can be used fordetermining vascular properties.

FIG. 8 represents the block diagram of the PCA-method and device. TheVUT-part provides pressure p_(cuff)(t), v_(AC)(t) and P_(n) as well asstate variables.

The method can also include the method for calculating the enhancedpressure signal p₊₊. Further, the method calculates PCA parameters likeCO, SV, SVV, PPV, arterial stiffness, etc and provides and displaysthese parameters and p₊₊. In addition, the method can obtainintermittent BP-readings (like systolic BP, mean BP and diastolic BP)from a standard NBP. Further, the method can be provided with anexcitation voltage from another device having an IBP-input in order toknow the scaling factor of such device.

The PCA method is now a multiport network or algorithm due to multipleinput signals in comparison to prior PCA with only one pressure or PPGinput.

The PCA method handles these multiple inputs. One embodiment of themethod is sequential mode, where p₊₊ is calculated first and then usedfor standard PCA. With that method information regarding vasomotoricchanges may be lost. Thus, the preferred embodiment uses linear andnon-linear multiport algorithms. In addition, these algorithms cancompute signal markers from the input time series, which can be forexample, areas under curves or part of the curves, duty cycles of thesignals, ratios (e.g. (mean BP−dia BP)/(sys BP−dia BP)), diastolicdecay, linear regressions of the signals and of the logarithmic signalstatistical moments, etc. These markers can be used for the computationof PCA-parameters along with anthropometric patient information (likeheight, weight, age, sex, etc.) and information obtained by theVUT-control system and its state variables. The computation can be madeout of multivariate polynomial equations. The weights of suchmultivariate equations can be either determined from physiologicala-priori information or be trained with machine learning methods using atraining set.

A height correcting system may also be used in conjunction with themethod for enhancing the blood pressure signal. Such a height correctingsystem may include a fluid-filled tube where the density of the fluidcorresponds to the density of blood. One end of the tube is placed atheart level and the other end is placed on the finger cuff. Afree-floating membrane, which prevents the fluid from escaping, could beattached at the heart end of the tube. A pressure sensor at the fingerend and connected directly to the fluid measures the hydrostaticpressure difference. The pressure sensor of the height correcting systemcan be constructed so that a frequency or digital signal at the sensorsite is produced and submitted to the overall control system.

Providing the enhanced or modified signals described above to otherdevices, e.g. commercially available patient monitors, would bedesirable, as all of them have an input for the standard IBP pressure.Thus, the non-invasive signal can be displayed on a screen and can bedistributed to other monitoring devices. Most patient monitors have aninterface for a pressure transducer in order to measure intra-arterialblood pressure (IBP). The IBP interface provides excitation voltage thatis used for scaling the blood pressure signal to voltage. The enhancedsignal may also be digitally distributed to further devices orcomputers, such as that described with respect to FIG. 9. The sameapplies for all calculated values like SV, CO, SVV, PPV, TPR, arterialstiffness, etc. as well as for the enhanced systolic, diastolic and meanBP values.

In order to determine the scaling range and factor, the patient monitorprovides an excitation voltage. Minimum and maximum pressures are knownfrom the specification. The excitation voltage can act as an input intothe present method and can transform p₊₊(t) or p_(brach)(t) to a voltagea₊₊(t) that emulates the output voltage of an intra-arterial transducer.The transformed and enhanced signal is transmitted to the other device.a₊₊(t) can be supplied by the analogue output of themircoprocessor/computer of the present device, an external DAC or byusing a PWM-output followed by a RC filter.

FIG. 9 is a block diagram illustrating an example computing device 200that may be associated with the system and method of the presentapplication. The computing device 200 may perform the methods of thepresent application, including the modification of signals, calculationof values, and execution of algorithms.

In a very basic configuration 201, computing device 200 typicallyincludes one or more processors 210 and system memory 220. A memory bus230 can be used for communicating between the processor 210 and thesystem memory 220.

Depending on the desired configuration, processor 210 can be of any typeincluding but not limited to a microprocessor (μP), a microcontroller(μC), a digital signal processor (DSP), or any combination thereof.Processor 210 can include one more levels of caching, such as a levelone cache 211 and a level two cache 212, a processor core 213, andregisters 214. The processor core 213 can include an arithmetic logicunit (ALU), a floating point unit (FPU), a digital signal processingcore (DSP Core), or any combination thereof. A memory controller 215 canalso be used with the processor 210, or in some implementations thememory controller 215 can be an internal part of the processor 210.

Depending on the desired configuration, the system memory 220 can be ofany type including but not limited to volatile memory (such as RAM),non-volatile memory (such as ROM, flash memory, etc.) or any combinationthereof. System memory 220 typically includes an operating system 221,one or more applications 222, and program data 224. For example, anapplication 222 may be designed to receive certain inputs from the PPGsystem and base decisions off of those inputs. For instance, theapplication may be designed to receive inputs from the PPG system, theNBP, and potentially other systems. As an output, the application 222may carry out any of the methods described herein above and provide ahigher fidelity BP signal.

Computing device 200 can have additional features or functionality, andadditional interfaces to facilitate communications between the basicconfiguration 201. For example, a bus/interface controller 240 can beused to facilitate communications between the basic configuration 201and one or more data storage devices 250 via a storage interface bus241. The data storage devices 250 can be removable storage devices 251,non-removable storage devices 252, or a combination thereof. Examples ofremovable storage and non-removable storage devices include magneticdisk devices such as flexible disk drives and hard-disk drives (HDD),optical disk drives such as compact disk (CD) drives or digitalversatile disk (DVD) drives, solid state drives (SSD), and tape drivesto name a few. Example computer storage media can include volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information, such as computer readableinstructions, data structures, program modules, or other data.

System memory 220, removable storage 251 and non-removable storage 252are all examples of computer storage media. Computer storage mediaincludes, but is not limited to, RAM, ROM, EEPROM, flash memory or othermemory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed bycomputing device 200. Any such computer storage media can be part ofdevice 200.

Computing device 200 can also include an interface bus 242 forfacilitating communication from various interface devices to the basicconfiguration 201 via the bus/interface controller 240. Example outputinterfaces 260 include a graphics processing unit 261 and an audioprocessing unit 262, which can be configured to communicate to variousexternal devices such as a display or speakers via one or more A/V ports263. Example peripheral interfaces 260 include a serial interfacecontroller 271 or a parallel interface controller 272, which can beconfigured to communicate with external devices such as input devices(e.g., keyboard, mouse, pen, voice input device, touch input device,etc.) or other peripheral devices (e.g., printer, scanner, etc.) via oneor more I/O ports 273. An example communication interface 280 includes anetwork controller 281, which can be arranged to facilitatecommunications with one or more other computing devices 290 over anetwork communication via one or more communication ports 282. Thecommunication connection is one example of a communication media.Communication media may typically be embodied by computer readableinstructions, data structures, program modules, or other data in amodulated data signal, such as a carrier wave or other transportmechanism, and includes any information delivery media. A “modulateddata signal” can be a signal that has one or more of its characteristicsset or changed in such a manner as to encode information in the signal.By way of example, and not limitation, communication media can includewired media such as a wired network or direct-wired connection, andwireless media such as acoustic, radio frequency (RF), infrared (IR) andother wireless media. The term computer readable media (or medium) asused herein can include both storage media and communication media.

Computing device 200 can be implemented as a portion of a small-formfactor portable (or mobile) electronic device such as a cell phone, apersonal data assistant (PDA), a personal media player device, awireless web-watch device, a personal headset device, an applicationspecific device, or a hybrid device that include any of the abovefunctions. Computing device 200 can also be implemented as a personalcomputer including both laptop computer and non-laptop computerconfigurations.

The physician is used to blood pressure values that are obtained atheart level. As the finger could be on a different hydrostatic level,the difference between finger and heart level could be corrected with awater filled tube between these two sites. Thus, a height correctingsystem may be applied in order to eliminate hydrostatic difference ofthe finger sensor and heart level.

While the invention has been described herein with relation to certainembodiments and applications, those with skill in the art will recognizechanges, modifications, alterations, and the like which still comewithin the spirit of the inventive concept, and such are intended to bewithin the scope of the invention as expressed in the following claims.

The invention claimed is:
 1. A method for determining a blood pressurecontour curve comprising: placing a cuff having a photo-plethysmographic(PPG) system over an artery in a human finger, the PPG system producinga PPG signal based on volume of the artery, the PPG system including atleast one light source and at least one light detector; modifying, by acomputing device, a component of the PPG signal having a frequencyhigher than a predefined threshold frequency by eliminating from the PPGsignal an undesired portion of the PPG signal and reconstructing the PPGsignal from the remaining portion of the PPG signal; the cuff pressurebeing controlled by one or more components of the modified PPG signal;and calculating, by the computing device, a blood pressure signal usingat least the modified component of the PPG signal.
 2. The method ofclaim 1 wherein a new blood pressure is calculated using the cuffpressure and the modified PPG signal.
 3. The method of claim 2 whereinmodifying a component of the PPG signal having a frequency higher than apredefined threshold frequency further comprises: separating the PPGsignal into a first component having a frequency higher than thepredefined threshold frequency and a second component having a frequencylower than the predefined threshold frequency; modifying the firstcomponent; and adding the modified first component to the secondcomponent to create a modified PPG signal; using the modified PPG signaland the cuff pressure to calculate the blood pressure signal.
 4. Themethod of claim 2 wherein the modification further includes calibratinga component of the blood pressure signal having a frequency higher thana predefined threshold frequency using a value obtained for bloodpressure by a sphygmomanometer placed on an artery in a human upper arm.5. The method of claim 2 wherein the modification further includesmultiplying a component of the blood pressure signal having a frequencyhigher than a predefined threshold frequency by a calibration factor,the calibration factor being calculated from a blood pressuremeasurement from a sphygmomanometer placed on an artery in a human upperarm.
 6. The method according to claim 2, wherein the threshold frequencyis about 0.3 Hz.
 7. The method according to claim 2, wherein thecalculation uses anthropometric parameters.
 8. The method according toclaim 2, further comprising calculating physiological parameters fromthe blood pressure contour curve.
 9. The method according to claim 8,wherein the parameters are calculated by using multiport algorithms. 10.The method according to claim 8, wherein the parameters are calculatedby using one or more markers of input signals.
 11. The method accordingto claim 1, wherein the reconstruction is calculated from the pulsatilepart of the remaining portion of the PPG signal.
 12. The methodaccording to claim 11, wherein the reconstructed PPG signal is$p_{Tn} = {{p_{T\; 0} + {g_{I} \cdot {\sum\limits_{0}^{n}P_{n}}} + {{g_{P} \cdot P_{n}}\mspace{14mu}{and}\mspace{14mu} P_{n}}} = {\int_{t_{n - 1}}^{t}{{v_{AC}(t)}{{\mathbb{d}t}.}}}}$13. The method according to claim 1, wherein the part of the PPG signalhaving the undesired portion of the PPG signal is below a predeterminedfrequency.
 14. A device for determining a blood pressure contour curvecomprising: a pressure cuff adapted to be placed over an artery in ahuman finger, the cuff including a PPG system having at least one lightsource and at least one light detector; a pressure sensor; a controllerfor controlling the pressure in the cuff; wherein the PPG systemproduces a PPG signal based on volume of the artery, a pressure signalis calculated by a computing device using the PPG signal, and thepressure signal is continuously applied to the cuff and finger; whereinthe computing device modifies a component of the PPG signal having afrequency higher than a predefined threshold frequency and calculates ablood pressure signal using the cuff pressure and the modified PPGsignal; and wherein the controller uses the one or more components ofthe modified PPG signal to control the pressure in the cuff.
 15. Thedevice according to claim 14, wherein the computing device receivescontrol state information from the controller.
 16. The device accordingto claim 14, wherein the computing device receives information from acalibration device.
 17. The device according to claim 14, wherein thecomputing device receives scaling information from another device. 18.The device according to claim 14, where the computing device receivesinformation from a hydrostatic correction system.
 19. The deviceaccording to claim 12, wherein the computing device receivesanthropometric information from the patient.
 20. The device according toclaim 14, wherein the computing device calculates physiologicalparameters from one or more input signals.